Hubble Reveals Ghostly Ring of Dark Matter
A ghostly ring of dark matter floating in the galaxy cluster ZwCl0024+1652, one of the strongest pieces of evidence to date for the existence of dark matter. Astronomers think the dark-matter ring was produced from a collision between two gigantic clusters.
Credit: ESA/Hubble

Updated at 2:34 pm EDT

Astronomers have discovered an enormous, ghostly ring of dark matter 5 billion light-years away--the most blatant evidence to date for the existence of a mysterious substance hidden throughout the universe.

Dark matter makes up a vast majority of gravity-exerting mass in the universe, while only about 10 percent is matter we can see and touch. If dark matter didn't exist, scientists say, galaxies like the Milky Way would have already flown apart from a severe lack of gravitational "glue."

Researchers pointed the aging but powerful Hubble Space Telescope toward a cluster of galaxies known as cluster ZwCl0024+1652. At first glance, the then-unknown ring looked like a ripple in a pond over the twinkling galactic cluster.

"I was annoyed when I saw the ring because I thought it was an artifact," said Myungkook James Jee of Johns Hopkins University.

But it wasn't a glitch, astronomers announced at a NASA press conference today.

The more Jee and others tried to remove the ring by tweaking the data, the more the ring-like anomaly stuck out like a sore thumb. "It took more than a year to convince myself that the ring was real," Jee said. "I've looked at a number of clusters and I haven't seen anything like this."

Because so much dark matter resides in the ring, astronomers said, it bends the light around it to create the ripple effect--dark matter's calling card. The complete findings will be detailed in an upcoming issue of the Astrophysical Journal.

The ring, 2.6-million light-years wide, formed when two huge clusters of galaxies slammed together in a head-on collision 6 to 7 billion years ago, puffing the mysterious matter outward, the astronomers figure. If the galactic hit-and-run had occurred outside of Earth's line-of-sight, the result might look more like the "Bullet cluster"--another cosmic impact site that astronomers view as strong evidence for dark matter.

Richard Massey, a Caltech astronomer not connected to the study, said that the finding is extremely important, especially combined with the Bullet cluster evidence. But he warned that the discovery still faces skepticism from other astronomers. "A lot of things can go wrong in producing an image," he said, explaining the shape could be produced within Hubble's camera itself.

Also, he said, the failure of Hubble's most powerful camera four months ago doesn't help. "Just as we were getting to the point to learn how to find dark matter, it breaks," Massey said.

Richard White, an astronomer with Space Telescope Science Institute in Maryland, said he also was initially skeptical about the ring of dark matter. "But it shows up in another Hubble camera's data as well," he said. "It's not as clear, but it's still there. We argue the ring has been seen twice now."

Unlike other dark matter discoveries, the ring is the first collection of dark matter that differs greatly from the distribution of ordinary matter.

In addition to using gravity to visualize the dark matter itself, the team also created computer simulations showing what happens when galaxy clusters collide. When the two clusters smashed together, they think, dark matter fell to the center and then sloshed outward. As it did so, gravity eventually slowed it down and condensed it into a large ring detected by astronomers.

"By studying this collision, we are seeing how dark matter responds to gravity," said Holland Ford, another Johns Hopkins astronomer on the team. "Nature is doing an experiment for us that we can't do in a lab, and it agrees with our theoretical models."

Finding dark matter is not easy because it doesn't shine or reflect light. So astronomers rely on gravity, which can bend the light of distant stars when enough mass is present, much like a lens distorts an image behind it. Thanks to the laws of physics, knowing how much light is bent tells astronomers how much mass is there. By mapping the gravity's "footprint," astronomers were able to create a picture of how the dark matter ring is distributed in the cluster.

In the image of the cluster, Jee said, "the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring. It's like looking at the pebbles on the bottom of a pond with ripples on the surface."